For the study, Owens, a University of British Columbia biologist,
and his colleagues analyzed the eyes of the fish, focusing on
light sensitive proteins called visual opsins. Each is most
sensitive to a particular wavelength of light. Humans, for
example, have three visual opsins sensitive to blue, green and
red light. They absorb light at slightly different wavelengths,
enabling us to see those three colors and others.

The scientists determined that the top part of Anableps'
eyes, the set that sticks out of the water, possess opsins
sensitive to green. The bottom half of the eyes, actually in the
water, are sensitive to yellow. The entire eye has genes
sensitive to ultraviolet, violet and blue light.

"This tells us that Anableps is more sensitive to yellow
light from the water and green light from the air," Owens said.
"We hypothesize that this functions to match their sensitivity
with the light available. The water Anableps lives in is
generally muddy (mangrove forests of northern South America) and
in this muddy water yellow light transmits best."

The unique visual system allows the fish to avoid a problematic
phenomenon Snell's Window," which occurs when you are underwater
while looking up out of the water. Due to the refraction of light
at the water's surface, after a certain angle you no longer see
out of the water, and instead see a reflection on the water's
surface. Thus, your field of vision is limited to about 96
degrees."

To compensate for this problem, certain other marine dwellers,
such as archerfish, have to mentally calculate refraction to find
the true position of objects they encounter. The "four eyed"
Anableps instead sees a broader angle.

The vision system and associated over water and under water
lifestyle comes at a price, though. As one might imagine, it's
not hard for predators to miss a bug-eyed fish skimming along the
surface. But Anableps is forever on the lookout, with
large areas of its brain devoted to vision.

The researchers suspect that Anableps used to just have
eyes suitable for the aerial environment. Over time, they think
the fish lost green sensitivity in the lower eye halves, gaining
yellow sensitivity there for better aquatic vision, particularly
in muddy yellow water.

Karen Carleton, an assistant professor in the University of
Maryland's Department of Biology, told Discovery News that "what
Dr. Owens and his colleagues are seeing is quite reasonable." She
said "it seems probable that Anableps has fine tuned"
its eyes "for its two visual tasks."

Shelby Temple of the University of Bristol's Visual Ecology Group
also supports the new findings, saying that they have "added yet
another example of a vertebrate that has the potential to have
different spectral sensitivity in different parts of its field of
view."

He said several fishes, amphibians, pigeons, other birds, and
certain primates, including humans, all possess what is known as
"intraretinal variability," meaning that variations in spectral
sensitivity exist across the retina, which is a delicate,
light-sensitive membrane lining the inner eyeball.

Temple concluded, "Now we just need to try and understand why so
many animals may be sensitive to different wavelengths of light
in different directions."